1. Field of the Invention
The present invention relates to methods for removing and sequestering carbon dioxide (CO2) from coal fired boilers and furnaces.
2. Description of Related Art
Bert Zauderer in U.S. Pat. No. 7,282,189, disclosed processes and methods whereby carbon dioxide (CO2) in the exhaust from the combustion in air of carbon in coal is physically separated from nitrogen (N2). Said separation is implemented by pressurizing the CO2 and N2 gas mixture with water all at the same pressure and in sufficient water quantity to dissolve the CO2 in the water. This is followed by exhaust of the N2, which is approximately 60 times less soluble in water and remains in the gas phase. The pressurized N2 expands in gas turbines in order to recover about 90% of the N2 compression power.
Said patent contains in column 15 lines 45 to 67 and column 16 lines 1 to 5 numerical examples on how to practices this separation. Specifically, compression of CO2 and N2 to 10 atmospheres (147 psi) reduces the amount of water needed to dissolve the CO2 by Henry's Law by a factor of 10 less than necessary to dissolve CO2 at atmospheric pressure. At ambient gas and water temperatures, 67 pounds of water per pound of gaseous CO2 are required compared to 670 pounds of water per pound of CO2 at atmospheric pressure. The N2 is then released from the mixing pressure vessel and expanded in a gas turbine to recover about 90% of the N2 compression power.
Column 17, lines 1 to 40, in said patent teaches further compression of the water-CO2 solution, which contains a small fraction of carbonic acid, to 1000 psi. This pressure equals that at 2150 feet below the surface, where injection of the carbonic acid solution into natural limestone geologic formations will form a calcium bi-carbonate solution that will permanently sequester the CO2. Column 16 lines 22 to 54 teach how this sequestration is implemented in a two-step process consisting of compression of CO2, N2 and water from atmospheric pressure at ambient temperatures to 147 psi. This is followed in step 2 by removal of the N2 and compression of the remaining CO2-water solution to 1000 psi for sequestration of the CO2 in limestone formations. Since this two step process consumes 25% of the power plants power output, another example in column 17, lines 1 to 40, teaches a one step process of compression from 14.7 psi to 1000 psi. This reduces the total sequestration power to 14% of the power plant's power output. In both the two-step and one step processes, a substantial amount of the power plant's output is derived from expanding the pressurized N2 to atmospheric pressure.
In either example in said Zauderer patent, the CO2 sequestration process is more efficient and far less costly than the U.S. Department of Energy's multi-billion dollar CO2 Sequestration Program that uses chemical separation, primarily with amines or ammonia, to separate the CO2 from the N2. The process consumes 26% of a plant's power output with one half of that sum consumed in the chemical separation step. One reason for using the chemical separation process is that the mol fraction by volume of CO2 in all coal power plants is 10% to 12% of the exhaust gases. In contrast, said Zauderer patent teaches that when only solid carbon is used as the fuel that is obtained from devolatilized coal char, or unburned carbon in coal ash, or anthracite culm, the CO2 in the exhaust is about 20% by volume. In addition, Zauderer's patent teaches combustion at a stoichiometric ratio of unity, namely with no excess air, which is possibly by firing said fuels in a refractory lined wall in an air-cooled slagging coal combustor, as per Zauderer's U.S. Pat. No. 4,624,191, which by reference is incorporated herein. In addition, the pollutants from coal combustion must be removed upstream of the CO2 separation and sequestration steps to prevent said pollutants from depositing in the sequestered underground CO2. Processes to accomplish the pollutant removals are disclosed in Zauderer's U.S. Pat. Nos. 7,553,463, 7,435,400, 7,282,189, 7,247,285, 6,722,295, 6,453,830, 6,048,510, and 4,765,258, which by reference are incorporated herein.
A major concern in the art of CO2 sequestration is the high cost of the process. While Zauderer's two U.S. patents on CO2 removal (U.S. Pat. No. 7,553,463 and U.S. Pat. No. 7,282,189) disclosed lower costs than all others disclosed in the prior art, further reduction are still possible to improve overall power output, reduce capital investment, and reduce operating costs. One low cost solution disclosed by Zauderer in U.S. Provisional Application No. 61/293,841, filed Jan. 11, 2010, which by reference is incorporated herein, is to incorporate CO2 separation with offsets, namely co-production of electricity with production of cementitious slag in the air-cooled slagging combustor that is converted from high ash char, or anthracite culm, or ash mixed with unburned carbon, and revenue from the sale of CO2 credits from sequestered CO2 from combustion and CO2 credits from avoided CO2 from Portland cement kilns that are replaced by cementitious slag. The potential revenue from these three products can be several times the revenue from electricity sales alone, which would far exceed the cost of CO2 separation and sequestration.
In the period from about June 2009 to date, the U.S. Department of Energy (DOE) awarded almost $2 billion dollars in R&D and Demonstration contracts that will be matched by over 50% from the private sector, to implement CO2 separation and sequestration contracts over most of this decade. Zauderer's January 2010 Provisional Application No. 61/293,841 stated that almost all the DOE awards were for variations of the costly, inefficient CO2 chemical separation processes. Also reported therein was a response to DOE's rejection within several weeks after receipt of Zauderer's August 2009 proposal for a 25 MW power plant demonstration to implement Zauderer's CO2 separation and sequestration processes, as disclosed in said Provisional application and U.S. Pat. No. 7,282,189. Despite meeting all the DOE solicitation requirements and offering to implement the demonstration plant at a very small fraction of the cost of the other awards, DOE rejected the proposal without a critical reviewer, as discussed in said Provisional application. Since January 2010 DOE has made scores of other CO2 awards, all of which are for chemical separation, except for a surprising $1 billion award in August 2010 in which DOE reversed its earlier decision to support the zero emission, with CO2 sequestration, coal gasification, combined gas turbine/steam turbine plant in Illinois, named Future Gen No. 1. It has been under study since 2003 by an international industry-government consortium. DOE cancelled the project in 2006 due to major cost overrun projections, but it was reinstated in 2008 by the current Administration. Then in August 2010 without an apparent new pre-announcement or competitive solicitation, DOE canceled the Future Gen 1 Project and replaced it with an equally rated 200+MW plant with zero emissions and with CO2 sequestration to be fired with oxygen, which removes the need for CO2 separation from N2 in the combustion gas exhaust. The DOE news release stated that a new O2 separation process based on an undisclosed membrane separation process offered by the American arm of Air Liquide, licensed to Babcock-Wilcox will be used. Zauderer in said U.S. Pat. No. 7,282,189, Column 16, lines 6 to 21 compares using pure O2 to burn coal compared to the CO2 process in said invention and states that removing the N2 would consume 2 MW, which is 20 times less than a standard O2 production system. The DOE press release asserts that the Air Liquide O2 process will not reduce the plant efficiency. However, intuitively a membrane separation process should cost much more that a particle baghouse, which costs about $100/kW. Also if the process is so cheap why is DOE committing $1 billion for a 200 MW power plant retrofit of which B&W will apparently receive $200 million for the O2 process.